1. Field of the Invention
The present invention relates to digital distance relays, and more specifically to a digital distance relay wherein each of N phases on duty among six phases has distance measuring elements in steps 1-3.
2. Description of the Prior Art
FIG. 1 shows an example of a digital distance relay in the prior art. In FIG. 1, the digital distance relay 1 receives an analog signal which is the output of a current transformer (hereinafter referred to as "CT") installed on transmission lines 2 to be protected and a potential transformer (hereinafter referred to as "PT") installed on bus lines 3 connected to the transmission lines 2 to be protected, and performs the distance measuring operation.
FIG. 2 shows an example of the constitution of the digital distance relay, and the analog signal from the CT, PT is introduced in an analog input member 4. The analog input member 4 comprises an input transformer 5, filters 6, a sample-and-hold circuit 7 (hereinafter referred to as "SH"), a multiplexer 8, an A-D converter 9 (hereinafter referred to as "AD"), a clock signal generator 10. A continuous signal inputted in the analog input member 4 is converted into a discrete digital signal.
Next, the digital signal is introduced in a computer member 11. The computer member 11 comprises a central processing unit 12 (hereinafter referred to as "CPU"), a setting circuit 13 being a sort of memory to store information for the reach setting of the distance relay and time setting of a timer, a read only memory 14 (hereinafter referred to as "ROM") to store software for the distance measuring operation, a randon access memory 15 (hereinafter referred to as "RAM") to store the input signal, the operation result etc., a digital input circuit 16 (hereinafter referred to as "DI") being a circuit to introduce external conditions such as a pallet condition of a circuit breaker, a digital output circuit 17 (hereinafter referred to as "DO") for outputting the trip command to the circuit breaker based on the decision result of the relay.
The digital signal from the analog input member 4 is transferred to the RAM 15 in the computer member 11 by a direct memory access circuit 18 (hereinafter referred to as "DMA"). The CPU 12 performs the operation processing of the digital signal according to the software stored in the ROM 14.
FIG. 3 is a flow chart illustrating the constitution of a conventional software to perform the operation processing of the digital signal. In FIG. 3, the operation from the starting up to the ending is performed during one sampling period. Here the term "sampling" means operation to take input data as performed in a digital protective relay, that is, operation to receive the analog signal from the PT or CT installed in the power system and to take the instantaneous value at every definite period (for example, at electric angle of 30 degrees in the rated frequency).
In FIG. 3, symbols 1ab, 1bc, . . . , 3c represent the processing of step 1 phase AB distance measuring operation, step 1 phase BC distance measuring operation, . . . , step 3 phase C distance measuring operation, respectively.
The digital distance relay is provided with the distance measuring elements to perform the measurement of steps 1-3 in each of six phases of AB, BC, CA, A, B, C, i.e. a total of 18 elements. The distance measuring operation is repeated 18 times simply in sequence of 1ab, 1bc, . . . , 3c corresponding to the 18 distance measuring elements per sampling.
Content of the distance measuring operation varies depending on the characteristics of individual distance measuring elements. For example, in the case of mho characteristics shown in FIG. 4, operation vector E.sub.OP and reference vector E.sub.POL are prepared according to following table. Decision is effected regarding whether or not the phase difference between the operation vector E.sub.OP and the reference vector E.sub.POL is within 90.degree., and if it is within 90.degree. the operation signal is outputted.
______________________________________ phase E.sub.OP E.sub.POL ______________________________________ AB V.sub.AB - Z.sub.F .multidot. I.sub.AB -V.sub.AB BC V.sub.BC - Z.sub.F .multidot. I.sub.BC -V.sub.BC CA V.sub.CA - Z.sub.F .multidot. I.sub.CA -V.sub.CA A V.sub.A - Z.sub.F (I.sub.A + K .multidot. 3I.sub.o) -V.sub.BC .angle.90.degree. B V.sub.B - Z.sub.F (I.sub.B + K .multidot. 3I.sub.o) -V.sub.CA .angle.90.degree. C V.sub.C - Z.sub.F (I.sub.C + K .multidot. 3I.sub.o) -V.sub.AB .angle.90.degree. ______________________________________ Z.sub.F : reach of mho characteristics K: zero sequence current compensation coefficient
Since the software of the distance measuring operation in the conventional digital distance relay is constituted as above described, the total operation processing time from the starting up of the operation to the ending thereof becomes approximately 18 times as large as the operation processing time per phase and step.
The operation time per phase and step is for the operation processing including multiplication and phase difference calculation as above described and therefore becomes relatively long. Consequently, if the total operation processing time should be contained within one sampling period, the sampling period must be made long. As a result, the operation finishing time of the distance relay cannot be made sufficiently fast.